Diagnosing Food Allergies continued...

Sometimes a food allergy diagnosis cannot be made solely on the basis of history. In that case, the doctor may ask the patient to go back and keep a record of the contents of each meal and whether he or she had a reaction. This gives more detail from which the doctor and the patient can determine if there is consistency in the reactions.

The next step some doctors use is an elimination diet. Under the doctor's direction, the patient does not eat a food suspected of causing the allergy, like eggs, and substitutes another food, in this case, another source of protein. If the patient removes the food and the symptoms go away, the doctor can almost always make a diagnosis. If the patient then eats the food (under the doctor's direction) and the symptoms come back, then the diagnosis is confirmed. This technique cannot be used, however, if the reactions are severe (in which case the patient should not resume eating the food) or infrequent.

Also thought to be helpful in diagnosing food allergies is an oral food challenge. If the patient or doctor fear a severe reaction, the suspected food can be given under medical supervision.

If the patient's history, diet diary, elimination diet, or oral food challenge suggests a specific food allergy is likely, the doctor may use tests that can more objectively measure an allergic response to food. One of these is a scratch puncture test, during which a dilute extract of the food is placed on the skin of the forearm or back. A skin puncture is made through the droplet with a needle and observed for swelling or redness that would indicate a local allergic reaction.

Skin tests are rapid, simple, and relatively safe although experts do not recommend making a food allergy diagnosis based on a skin test alone. A patient can have a positive skin test to a food allergen without experiencing allergic reactions to that food. A doctor diagnoses a food allergy only when a patient has a positive skin test to a specific allergen and the history of these reactions suggests an allergy to the same food.